As is well known in the art, in medical diagnosis and treatment, it is often desirable to quantitatively measure over time changes in body position. This has conventionally been done by having an attendant record changes in posture.
As is well known in the art, in medical diagnosis and treatment, it is often desirable to quantitatively measure over time the respiratory air volume or pulmonary ventilation. This has conventionally been done by having the patient or subject breathe into a mouthpiece connected to a flow rate measuring device. Flow rate is then integrated to provide air volume change.
There are, however, several drawbacks and disadvantages associated with employing a mouthpiece. A mouthpiece is difficult to use for long term subject monitoring, especially for ill, sleeping, or anesthetized subjects. Further, it is uncomfortable for the subject, tends to restrict breathing, and is generally inconvenient for the physician or technician to use.
As is also well known in the art, there are qualitative respiration monitors available that do not require a mouthpiece. Illustrative are the systems disclosed in U.S. Pat. Nos. 3,831,586 and 4,033,332. Although the noted systems eliminate most of the disadvantages associated with a mouthpiece, the systems do not, in general, provide an accurate measurement of air volume. Further, the systems are typically only used to signal an attendant when a subject's breathing activity changes sharply or stops.
Another means for quantitatively measuring respiratory or lung volume is to measure the change in size of the rib cage and abdomen, as it is well known that lung volume is a function of these two parameters. A number of systems and devices have been employed to measure the change in size (i.e. Δ circumference) of the rib cage and abdomen, including mercury in rubber strain gauges, pneumobelts, magnetometers, and respiratory inductive plethysmograph (RIP) belts. These systems, in general, provide an accurate measurement of air volume when body position does not change.
In practice, respiratory magnetometers and RIP belts are primarily used to measure the change in size of the rib cage and abdomen. As is well known in the art, respiratory magnetometers consist of tuned pairs of electromagnetic coils or magnetometers; one coil being adapted to transmit a specific high frequency AC electromagnetic field (i.e. transducer) and the other coil (i.e. receiver) being adapted to receive the field. To measure the anteroposterior diameter of the rib cage, a first coil, e.g., transducer, is typically placed over the sternum at the level of the 4th intercostal space and the second coil (of the pair) is placed over the spine at the same level. To measure the anteroposterior diameter of the abdomen, a third coil is typically placed on the abdomen at the level of the umbilicus and a fourth coil (of the pair) is placed over the spine at the same level.
Over the operational range of distances, the output voltage is linearly related to the distance between a pair of coils; provided, the axes of the coils or magnetometers remain parallel to each other. As rotation of the axes can change the voltage, the transducer and receiver coils must be secured to the skin in a parallel fashion and rotation due to the motion of underlying soft tissue that must be minimized.
A potential limitation of the use of such coils or magnetometers is presented in environments that contain large metal structures or electric motors. Such devices produce extraneous electromagnetic fields and consequently affect the magnetometer voltage output.
Displacement (i.e. motion) of the rib cage can be directly assessed. Diaphragm displacement cannot be measured directly, but since the abdominal contents are essentially incompressible, caudal motion of the diaphragm relative to the pelvis and the volume it displaces is reflected by outward movement of the anterolateral abdominal wall.
The “two-degrees-of-freedom” model embraced by most in the field holds that the volume displacement of the respiratory system, i.e. tidal volume (VT), is equal to the sum of the volume displacements of the rib cage and abdomen, i.e.VT=αRC+βAb  Eq. 1where:RC and Ab represent linear displacements of the rib cage and abdomen, respectively; and α and β represent volume-motion coefficients.
As is well known in the art, RC and Ab linear displacements are converted to RC and Ab volume displacements when multiplied by the α and β volume-motion coefficients.
It is well established that the use of the noted “two-degrees-of-freedom” model can provide an estimate of VT that is within 10% accuracy of ventilation measured at the mouth; provided, the subject is confined to one body position.
Two different approaches primarily used for determining the necessary volume-motion coefficients of the rib cage and abdomen are the isovolume technique and the multiple linear regression technique. In the isovolume technique, the subject first performs an isovolume maneuver, shifting volume back and forth between the rib cage and abdominal compartments while holding the glottis closed, whereby there is no net volume change of the system. Since VT equals zero, Equation 1 can be modified as follows:RC=(−β/α)Ab  Eq. 2
On a graph of rib cage and abdomen signals, the slope of the isovolume line is equal to the ratio −β/α.
In practice, the gains of the rib cage and abdomen signals are often adjusted, whereby the slope of the isovolume line equals one. The rib cage and abdomen displacements are thus equal for any volume change. The two signals can then be directly summed to provide volume.
The isovolume method is based on the assumptions that displacements of the surfaces of the rib cage and abdomen are representatively sampled at the measured location, and are similar during isovolume efforts and spontaneous breathing. Since volume-motion coefficients change with posture, the isovolume calibration must be repeated in each body position.
Computer-assisted regression techniques, such as multiple linear regression, are used to determine volume-motion coefficients by solving a matrix of multiple simultaneous equations of changes in chest wall dimensions and lung volume. An advantage of these techniques is that no special calibration maneuver is required to generate volume-motion coefficients.
A limitation of any approach that uses chest wall motion to assess ventilation is, however, that the overall volume change of the chest wall being measured includes not only changes in lung volume, but also blood volume shifts into and out of the thoracoabdominal cavity. This can occur when the respiratory system is subjected to large pressure changes, or with changes in posture (e.g. between supine and upright position).
Another limitation is related to distortion that can occur within the rib cage or abdomen (e.g. between the upper and lower rib cage or between the lower transverse and AP rib cage).
It is proposed to use the following “three-degrees-of-freedom” model to determine tidal volume, i.e.VT=αRC+βAb+γXi  Eq. 3where:RC and Ab represent linear displacements of the rib cage and abdomen, respectively; Xi represents the Δ distance between the xiphoid and the pubic symphysis; and α, β and γ represent volume-motion coefficients for RC, Ab and Xi.
There are, however, similarly several drawbacks and disadvantages associated with the ‘three-degrees-of-freedom” model. A major drawback is that the ‘three-degrees-of-freedom” model reflected in Eq. 3 above is still limited in accuracy to about 15% of actual ventilation in individuals who are doing freely moving postural tasks, such as bending, sitting or standing, due to spinal flexion.
It would thus be desirable to provide an improved method and associated system for determining tidal volume (or pulmonary ventilation) that substantially reduces or eliminates the drawbacks and disadvantages associated with conventional methods and systems that are employed to determine pulmonary ventilation.
As is well known in the art, assessing changes in body position is an integral component of the diagnosis and treatment of sleep apnea. It is important to ascertain body position during sleep as sleep disordered breathing may be position dependent.
Sleep apnea is a disorder that affects 5-10% of the population. The supine sleep position often favors the development of apneas and hypopneas. Body position is often documented by a technician in a sleep lab. Drawbacks to performing overnight polysomnography in a sleep lab include sleep fragmentation brought about by an individual sleeping in a foreign setting outside the home and the need for a technician to attend the procedure and document changes in body position.
It is important to have non-invasive methods to ascertain body position which do not require the presence of an attendant. This would allow for remote assessment of body position.
Apneas and hypopneas are detected by analyzing breathing pattern during sleep. Apneas are defined as a cessation of airflow for greater than 10 seconds and a hypopnea is defined as a greater than 50% reduction in airflow lasting at least 10 seconds and associated with oxyhemoglobin desaturation of 3% or greater. Currently, numerous methods are utilized to determine apneas and hypopneas including measurement of motion of the rib cage and abdomen. These methods do not accurately measure the amount of air inhaled or exhaled (tidal volume; VT). This failure in methodology, which is common to all commercially available polysomnography systems, makes detecting hypopneas subjective and variable among scorers.